Myogenic cells grown in culture retain many of their differentiated, cell surface associated properties. Using our recently developed methods for systematically altered the myoblast lipid composition, we have obtained evidence suggesting an intimate, direct lipid involvement in myoblast fusion. From these and other observations we have developed a working hypothesis for myoblast fusion in which fusion is a lipid mediated event whose rate is modulated by the physical state of the lipids. The overall objective of this proposal is to test and develop this hypothesis. First, we will develop a useful fluorescence assay of myoblast fusion. We will then explore the analogy between the fusion of myoblasts and lipid vesicles. This will be done using physical techniques, chemical cross-linking, and systematic perturbations of the lipid compositions. One objective is to determine which model fusion system, if any, is most relevant to the myoblast fusion mechanism. Special emphasis will be placed on the role of lipid phase separations and defect structures in modulating the fusion rates in model system and in the myoblasts. This entails determining the asymmetry and principal lipid subclasses the myoblasts. We will determine the lipid asymmetry and lipid subclasses using previously published chemical procedures. The phase equilibria will be studied primarily using paranaric acid fluorescence and differential scanning calorimetry. The information we obtain in model lipid vesicles will be synthesized into a statistical mechanical description of fusion using the results of our previous description of lipid bilayers. The goal is to predict the activation energies of the fusion rates.